The image transmission technology, especially the technology on transmitting the compressed encoded images, has been wildly used in videoconferences and videophones. In real-time bi-directional video communication, such as videoconference, the video information is compressed into code stream data for transmission according to international standards. H.26X standards proposed by ITU-T, such as H.261 and H.263, are common ones used for encoding images at present. In the H.26X standards, the format of digital video is defined as Common Intermediate Format (CIF) and Quarter Common Intermediate Format (QCIF) that are unified image formats for videoconferences. These image formats can be used in different international television standards, such as PAL, NTSC and SECAM etc.
Taking the CIF format as an example, the structure of digital video formats defined in the H.26X standards will be briefly described. FIG. 1 and FIG. 2 respectively show the constitution of the H.261 CIF format and the H.263 CIF format. As shown in FIG. 1 and FIG. 2, each image frame is divided into multiple Groups of Blocks (GOBs), and each GOB is divided into multiple macro-blocks (MB). The MB can be divided further, but since it does not relate to the present invention, it will not be further described. In accordance with the image formats, code stream data of a compressed encoded image formed according to the H.26X standards is arranged into multiple layers during transmission. Among these layers, the image layer consists of an image layer header and data in the GOB layer, the GOB layer consists of a GOB layer header and data in the MB layer, the MB layer consists of a MB layer header and data in the next layer, and so on. Since the redundancy of a compressed code stream is low, the image quality at the receiving end is greatly affected by transmission channel error codes produced in transmission.
In general, an error code detection manner is used at the receiving end, such as the Forward Error Correction (FEC). When an error code is detected, the displaying image at the receiving end is frozen, namely, the process of decoding image data to video data which can be broadcast directly is paused. At the same time, a FastUpdate command is sent from the receiving end to the transmitting end through conference control. After receiving the FastUpdate command, the transmitting end encodes the current frame through encoding within frame and sends the encoded frame, and sets the location of Freeze Release bit in code streams as validness. After detecting the location of Freeze Release bit in code streams is set as validness, the receiving end cancels display freezing, and decodes image data to video data which can be broadcast directly continually.
The disadvantage of the prior art is that even there are few error codes, freezing images and intra-frame encoded images with low quality will frequently appear during decoding image data. Especially in a multipoint videoconference, when a terminal is broadcasting, if error codes appeared in lines of a certain receiving terminal, intra-frame encoded images with low quality will frequently appear in all of the terminals used in the videoconference owing to the above-mentioned processing manner for error codes. Usually, this is unacceptable for users.